[9] RFR (M): 8063137: Never-taken branches should be pruned when GWT LambdaForms are shared

[Vladimir Kozlov]

Nice! At least Hotspot part since I don't understand jdk part :)

I would suggest to add more detailed comment (instead of simple "Stop 
profiling") to inline_profileBranch() intrinsic explaining what it is 
doing because it is not strictly "intrinsic" - it does not implement 
profileBranch() java code when counts is constant.

You forgot to mark Opaque4Node as macro node. I would suggest to base it 
on Opaque2Node then you will get some methods from it.

Thanks,
Vladimir

On 1/16/15 9:16 AM, Vladimir Ivanov wrote:
> http://cr.openjdk.java.net/~vlivanov/8063137/webrev.00/hotspot/
> http://cr.openjdk.java.net/~vlivanov/8063137/webrev.00/jdk/
> https://bugs.openjdk.java.net/browse/JDK-8063137
>
> After GuardWithTest (GWT) LambdaForms became shared, profile pollution
> significantly distorted compilation decisions. It affected inlining and
> hindered some optimizations. It causes significant performance
> regressions for Nashorn (on Octane benchmarks).
>
> Inlining was fixed by 8059877 [1], but it didn't cover the case when a
> branch is never taken. It can cause missed optimization opportunity, and
> not just increase in code size. For example, non-pruned branch can break
> escape analysis.
>
> Currently, there are 2 problems:
>    - branch frequencies profile pollution
>    - deoptimization counts pollution
>
> Branch frequency pollution hides from JIT the fact that a branch is
> never taken. Since GWT LambdaForms (and hence their bytecode) are
> heavily shared, but the behavior is specific to MethodHandle, there's no
> way for JIT to understand how particular GWT instance behaves.
>
> The solution I propose is to do profiling in Java code and feed it to
> JIT. Every GWT MethodHandle holds an auxiliary array (int[2]) where
> profiling info is stored. Once JIT kicks in, it can retrieve these
> counts, if corresponding MethodHandle is a compile-time constant (and it
> is usually the case). To communicate the profile data from Java code to
> JIT, MethodHandleImpl::profileBranch() is used.
>
> If GWT MethodHandle isn't a compile-time constant, profiling should
> proceed. It happens when corresponding LambdaForm is already shared, for
> newly created GWT MethodHandles profiling can occur only in native code
> (dedicated nmethod for a single LambdaForm). So, when compilation of the
> whole MethodHandle chain is triggered, the profile should be already
> gathered.
>
> Overriding branch frequencies is not enough. Statistics on
> deoptimization events is also polluted. Even if a branch is never taken,
> JIT doesn't issue an uncommon trap there unless corresponding bytecode
> doesn't trap too much and doesn't cause too many recompiles.
>
> I added @IgnoreProfile and place it only on GWT LambdaForms. When JIT
> sees it on some method, Compile::too_many_traps &
> Compile::too_many_recompiles for that method always return false. It
> allows JIT to prune the branch based on custom profile and recompile the
> method, if the branch is visited.
>
> For now, I wanted to keep the fix very focused. The next thing I plan to
> do is to experiment with ignoring deoptimization counts for other
> LambdaForms which are heavily shared. I already saw problems caused by
> deoptimization counts pollution (see JDK-8068915 [2]).
>
> I plan to backport the fix into 8u40, once I finish extensive
> performance testing.
>
> Testing: JPRT, java/lang/invoke tests, nashorn (nashorn testsuite, Octane).
>
> Thanks!
>
> PS: as a summary, my experiments show that fixes for 8063137 & 8068915
> [2] almost completely recovers peak performance after LambdaForm sharing
> [3]. There's one more problem left (non-inlined MethodHandle invocations
> are more expensive when LFs are shared), but it's a story for another day.
>
> Best regards,
> Vladimir Ivanov
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8059877
>      8059877: GWT branch frequencies pollution due to LF sharing
> [2] https://bugs.openjdk.java.net/browse/JDK-8068915
> [3] https://bugs.openjdk.java.net/browse/JDK-8046703
>      JEP 210: LambdaForm Reduction and Caching
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